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copper composites
Diamond & Related Materials 98 (2019) 107467
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Effect of surface roughening on the interfacial thermal conductance of diamond/copper composites Xinzhi Wua, Liyang Lia, Wei Zhanga, Manxin Songa,b, Wulin Yanga, Kun Penga,
T
⁎
a
College of Materials Science and Engineering, Hunan University, 410082 Changsha, China Hunan Engineering Technology Research Center for Microwave Devices and Equipment, Hunan Aerospace Chengyuan Precision Machinery Co., LTD, Changsha 410205, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface Interface Thermal conductivity Diamond Composites
Understanding the mechanism of interfacial heat transfer is the key to improved thermal conductivity of diamond/copper composites. Diamond particles are etched via molten salt to obtain different surface roughness and are subsequently coated with dual layers of tungsten and copper (Cu). Then, diamond/Cu composites are prepared by hot pressed sintering. When the (111)D surface roughness of the diamond is 11.1 μm, a maximum thermal conductivity of 602 W·m−1·K−1 is obtained, which is 12% higher than that of un-etched diamond/Cu composites. The roughness significantly increases the coupling area between the diamond and the Cu matrix, providing more heat transfer channels and a secondary heat transfer process at the diamond/matrix interface. The preparation of the high-thermal-conductivity composites is thus enhanced by controlling the surface roughness of the diamond particles.
1. Introduction
increasing the interfacial bonding area. The effect of interfacial area on the thermal boundary conductance has recently attracted attention, where it was found by molecular dynamics analysis that surface roughness could improve the thermal conductivity of composites [24]. Tan [26] and O. Beffort [27] have found that dendrite and rod-shaped carbide transition layers enhanced the thermal conductivity of diamond/aluminum (Al) composites; this was attributed to better interface bonding. However, the formation of dendrite and rod-shaped carbide transition layers also increased the interfacial area, which may have also contributed to the thermal conductivity. The thermal properties of diamond/Al composites were improved by roughening the diamond surfaces with iron‑manganese powders and nitrogen annealing [28,29]. In addition to the increased contact area between the two phases, annealing of the diamond surface led to (111) facets on the (100) surface that facilitated nucleation of carbides. However, the effect of diamond surface roughness on the heat coupling area and the thermal conductivity of diamond/Cu composites has not been systematically investigated. Here, diamond particles with different surface roughness were prepared and used in diamond/Cu composites. The thermal conductivity was improved by controlling the diamond surfaces. The effects of changes in the composite interface on heat transfer is discussed in detail.
Diamond/Cu composites have been considered the first candidate for the next generation electronic packaging materials for their excellent thermal conductivity in theory [1,2]. However, poor bonding [3] and high interfacial thermal resistance [4] between the diamond and copper (Cu) has resulted in poor thermal conductivity of the composites relative to theoretical values [5]. To overcome this problem, titanium, tungsten, chromium, and boron were coated on the diamond surface [5–13], or introduced into the Cu matrix [14–17], to form carbides. However, because of the low thermal conductivities of the carbides [18] and the difficulty of controlling the thicknesses [19,20], it was difficult to achieve the ideal thermal conductivity in the composites. The focus in these studies was to increase the interfacial thermal coupling between the two phases by the formation of carbides on the diamond surface. However, they ignored contributions from a number of interfacial heat transfer channels to the overall thermal conductivity. The interfacial heat transfer across the interface between two dissimilar materials [21–25] plays a key role in the thermal conductivity of composites. Heat transfer through the interface is realized by the coupling between carriers in the two phases. Therefore, the heat transfer efficiency is determined by the magnitude of the coupling and interfacial heat transfer channels. The former can be improved by the introduction of a transition layer, while the latter can be realized by ⁎
Corresponding author. E-mail address: [email protected] (K. Peng).
https://doi.org/10.1016/j.diamond.2019.107467 Received 25 April 2019; Received in revised form 26 June 2019; Accepted 10 July 2019 Available online 11 July 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. SEM micrographs of the diamond particle etched at different temperature: (a, a1, a11) Un-etched, (b, b1, b11) 750 °C, (c, c1,c11) 800 °C, (d, d1, d11) 850 °C,
(15 ml·L−1). The pH was then adjusted to 12 by NaOH (10 mol·L−1). The diamond particles were reacted at 30 °C for 12 h, and then washed and dried to obtain double-coated samples. During the experiment, we tested the quality change of the diamond before and after each coating. It should be pointed out that the experimental conditions of the different samples are identical except for the change of the etching temperature.
2. Experimental 2.1. Surface modification of diamond particles Synthetic diamond particles (HWD-40) with an average size of 400 μm (40–45 mesh) were purchased from Henan Huanghe Whirlwind International Co., Ltd. They were complete cubo-octahedrons with a nitrogen content of 150–170 ppm [30] and had a 1500-W·m−1·K−1 thermal conductivity. The particles were cleaned with acetone and hydrochloric acid to remove grease from the surfaces. They were then etched with molten potassium nitrate at a 1:10 diamond/potassium nitrate weight ratio, followed by heating at 750 °C, 800 °C, or 850 °C for 15 min. After cooling to room temperature, the consolidated potassium nitrate was dissolved with deionized water, and residual impurities were removed by ultrasonic cleaning. And the detail experimental process was described in our previous paper [31]. A layer of tungsten (W) was coated onto the diamond particles via vacuum diffusion. The volume ratio of particles to high-purity W powder with an average size of about 75 μm (200 mesh) was 1:1. The mixed powders were placed in a corundum crucible and the diffusion treatment was performed in a vacuum at 1.0 Pa, 900 °C for 150 min. After the tube furnace returned to room temperature, the W-coated diamond particles were extracted from the mixed powder via ultrasonic cleaning and mechanical sieving. Then, the W layer was coated with a layer of Cu via electroless plating. The typical bath chemistry includes: Cu2SO4·5H2O (15 g·L−1), Na2EDTA·2H2O (30 g·L−1) and HCHO (37%)
2.2. Preparation of diamond/Cu composites High purity copper powder (Hunan CSBLSTAR Metal Materials Co. Ltd. of China) with mean size of 45 μm (400–450 mesh) was used as the composite matrix. The Cu powders were mixed with the surface-modified diamond particles (about 38 vol%) and then placed in a graphite mold that was held in a 900 °C vacuum hot press sintering furnace (JY160-00) at a pressure of 70 MPa for 60 min in vacuum atmosphere (about 1 × 10−1 Pa). 2.3. Characterization Scanning electron microscopy (SEM) images and elemental mapping were acquired with a field-emission scanning electron microanalyzer (TESCAN MAIA3), equipped with an energy-dispersive X-ray analyzer. Raman spectra were obtained with a spectrometer (Renishaw 2000) and 514-nm laser excitation. Three-dimensional (3D) model maps and the surface roughness Rz with an accuracy of ± 0.5 μm were 2
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Fig. 2. 3D microscopic map of diamond particle etched at 800 °C for 15 min, (a) (111)D surface of diamond; (b) (100)D surface of diamond.
Fig. 3. Surface roughness change of (100)D and (111)D surfaces.
Fig. 4. Raman spectra of the diamond with different etching conditions.
acquired with a VHX-2000C super-high magnification zoom lens on a 3D microscope. The phase structure of the diamond particles was confirmed with X-ray diffraction (SmartLab, Rigaku Japan) with a CuKα radiation source. The samples used to observe the interface of the diamond/Cu composites by electrochemical polishing method. The electrolyte was H3PO4 with a volume fraction of 70%, the current density was 1A/cm2 and the corrosion time was 10 min.
The mass-average thickness of the coating for smooth diamond particles was calculated by a gravimetric method. The thermal conductivity of the diamond/Cu composites has been obtained by the formula: K = α ∙ c ∙ ρ, Where K is the thermal conductivity, α is the thermal diffusivity with an accuracy of ± 3% obtained via laser-flash analyzer (LFA467), c is the theoretical specific heat capacity obtained by the rule of mixture (ROM), ρ represents the density of diamond/Cu 3
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Fig. 5. Morphologies of diamond particles after tungsten coating with different etching degrees; (a) Un-etched, (b) 750 °C, (c) 800 °C, (d) 850 °C; (e), (f), (g), (h) are higher magnification images at 800 °C.
Fig. 6. Morphologies of diamond particles after tungsten and copper coating with different etching degrees; (a) Un-etched, (b) 750 °C, (c) 800 °C, (d) 850 °C; (e) and (f) are higher images at 800 °C.
morphologies of the raw and etched particles are shown in Fig. 1. The raw particles were cubo-octahedral crystallites consisting of six square {100} faces and eight hexagonal {111} faces with smooth surfaces. Fig. 1(b–d) shows the morphologies of particles etched at 750 °C, 800 °C, and 850 °C, respectively. The subscripts represent different magnifications. The particles etched at low temperature maintained the cubo-octahedron shape, but many triangular and square structure pits were observed on the {111} and {100} planes. It can be further seen
composites obtained by the Archimedes drainage method.
3. Results and discussion 3.1. Surface modification of diamond particles The synthetic diamond particles were etched with potassium nitrate at different temperatures (750 °C, 800 °C, and 850 °C), respectively. The 4
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Fig. 7. Morphology and elemental mapping of the coated diamond particle: (a–d) W coated; (e–h) W and Cu coated.
shown in Fig. 2 for particles etched at 800 °C for 15 min. The etch pits of triangles and squares were distributed on the (111) and (100) planes, respectively. Through modeling of the instrument, a 3D map containing specific etch pit depths (surface roughness) was obtained. The relationship between surface roughness Rz and etching temperature is shown in Fig. 3. Un-etched particles had a model height difference of 0.8–1.7 μm matching Fig. 1(a11) because of intrinsic defects. The pit depth on the (111) plane was about 1.5 times that on the (100) plane because the (111) plane was more easily etched given its lower chemical resistance [32]. Hence, the (111) roughness was greater under the same etching conditions, and it increased with temperature. Fig. 4 shows micro laser Raman spectral results. The diamond particles with various degrees of etching had only one peak at 1332.2 cm−1, which was associated with the sp3 structure, and was consistent with raw diamond. Thus, it could be inferred that the molten salt etching did not cause graphitization of the diamond or affect its intrinsic thermal conductivity. The W-Cu dual-layer coatings can increase the homogeneity and density of composite materials [33–35], and allow the Cu matrix to penetrate the etch pits more easily. The surfaces of the W-coated diamond particles were characterized with SEM, as shown in Fig. 5. The etched pits were coated outside and inside with a relatively uniform and continuous layer. The microstructure was still clearly visible, which provided a favorable basis for examining the effects of interfacial area on heat transfer. The surfaces of the Cu-W dual-layer-coated diamond particle were characterized with SEM, as shown in Fig. 6. By comparing Fig. 5 and Fig. 6, it was found that the Cu layer was relatively thick and that the etch pits had been filled. Secondly, according to Fig. 5(h) and Fig. 6(f), the Cu and W coating are dense, rough and continuous.
Fig. 8. X-ray diffraction of the diamond particles; (a) W coated, (b) W and Cu coated.
from the high magnification images, the size of the etched pits increased with the temperature. The formation of the pits was attributed to the atomic configuration of the diamond surface, distortion energy, and functional oxygen groups [31,32]. For the sample etched at 850 °C, the etching pits merged into large pits, and the edges and corners of the particles were severely etched. To obtain the quantitative roughness of the etched diamond surfaces, optical morphologies and the 3D model maps were used, as
Table 1 Thermal diffusivity α, specific heat capacities c, densities ρ, closed porosity ε, diamond volume fraction νd and thermal conductivity of the Cu matrix and the diamond/Cu composites. Samples Cu Un-etched 750 °C-etched 800 °C-etched 850 °C-etched
α (mm2·s−1)
c (J·g−1·m−1)
ρ (g·cm−3)
ε (%)
νd (%)
K (W·m−1·K−1)
111.0 191.5 200.5 214.2 155.9
0.395 0.42 0.42 0.42 0.42
8.90 6.75 6.71 6.72 6.75
0.9 1.6 2.5 2.6 2.5
– 38 38 38 38
390 539 557 602 439
5
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Fig. 9. Microstructures of diamond/Cu composites with different etching degree: (a) Un-etched, (b) 750 °C, (c) 800 °C, (d) 850 °C.
treatment. In order to quantitatively estimate the thickness of the coating, the equivalent mass-average thickness (h, nm) of ideal and smooth W coating were calculated by the following formula [36]:
h=
mw ·103 ρw ms
(1)
where mw represent the tungsten mass in the coating and m is the diamond mass. ρw is the tungsten density (19.3 g/cm3), s is the specific geometric surface area of diamond particles, which can be calculated by the following formula [36]:
s=
6 ρd Dψ
(2)
where ρd represents the diamond density (3.52 g/cm3), D is the average size of the diamond particle, the correction factor ψ of cubo-octahedron is 0.905 [36], s = 0.0047 m2/g can be calculated by the equation, and then the mass-average thickness of W coating can be obtained. The mass-average thickness of Cu can also be estimated by this method. The thickness of W and Cu coating on the un-etched diamond particles is about 170 nm and 2.4 μm, respectively, and they have a decrease with the increase of surface roughness.
Fig. 10. TC and inferred ITC of diamond/Cu composites with different etching degrees.
Fig. 7 shows the morphology and elemental distribution of the Wcoated and Cu- W dual-layer-coated diamond particles. The W and Cu were uniformity coated on the diamond surfaces. However, the elemental mapping cannot detect the existence of the W because of the thick coating of Cu. From the XRD patterns in Fig. 8, the elemental W was present as W, W2C, and WC following vacuum diffusion, whereas the elemental Cu was present as crystals. No metal oxides were detected throughout the entire process, and no diamond graphitization occurred during the heat
3.2. Microstructure of diamond/Cu composites The composite material was treated with electrochemical corrosion to more intuitively observe the interfacial structure between the diamond and the Cu matrix. Fig. 8 shows the interfacial structure of the diamond/Cu composites after different degrees of etching after W and Cu coating. A good combination of diamond and matrix was 6
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Fig. 11. Schematic diagram of etching pits and interfacial heat flux: (a-b) etching and interface coupling area; (c) Flat interface, (d) Rough interface, (e) Singlecoupling at the pits.
800 °C for 15 min. However, as the degree of etching increased, the thermal conductivity decreased significantly to 439 W·m−1·K−1, which was lower than that of the raw diamond/Cu composite. These changes could be attributed to the diamond particle surfaces at the composite interfaces. To analyze the thermal conductivity of particle-reinforced metal matrix composites, the Hasselman-Johnson (H-J) [37] model considers the effects of particle size, volume fraction, and interfacial thermal conductance. It has a wide range of applications, and can be expressed as follows:
maintained. The Cu matrix was relatively compact without porosity. Moreover, etching pits were not destroyed by hot pressing, which also may have increased the matrix/diamond bonding and strengthened the mechanical properties of the composite. 3.3. Thermal conductivity of the diamond/Cu composites The thermal properties of Cu and the diamond/Cu composites are listed in Table 1. The thermal diffusivity was a three-point average, and the calculation of the specific heat capacity did not include the coating. The theoretical density (ρ⁎) of the ideal nonporous composites was obtained by the formula [36]:
ρ∗ =
( (
1+C 1−x ρd
+
x ρc
+
C ρm
(3)
ρ ρ∗
(4)
νd =
1−x ρ · 1 + C ρd
(5)
K
) )
K
K
(6)
K is the thermal conductivity, and the subscripts c, d, and m represent composite materials, reinforcing particles, and matrix, respectively. V and a represent the real volume fraction and the radius of the diamond particles in the filler, respectively, and hc is interfacial thermal conductance. By substituting the experimental value for K into Eq. (1), hc can be calculated. The trend was consistent with the change in thermal conductivity of the diamond/Cu composite, with a hc maximum of 2.23 × 107 W·m−2·K−1 after 800 °C etching. And the results are shown in Fig. 10. The interfacial thermal conductance calculated in the H-J model is a parameter that characterizes the interface combination. The H-J equation is derived from smooth-filled particles, because the surface roughness is much smaller than the particle size, the model can be used for approximate estimation of interfacial thermal conductance were effective. Here, the diamond particle sizes and fractions were kept the same, and the surfaces were modified by the same method used to guarantee a tight coupling with the copper matrix. Thus, the uniformity of the interface per unit area was unchanged, and the only change was the diamond surface roughness. Therefore, the heat transfer per unit area (heat coupling) at the interface could be considered as a constant, and the changes in interfacial thermal conductance were caused by changes in the interfacial coupling area. Interfacial heat transfer in the diamond‑copper composites was attributed to the synergy of phonons and electrons [22,38]. Fig. 11(a–b) shows the Schematic diagram of etching pits and the coupling area of
where C is the mass ratio of the Cu to the coated diamond, and x is mass fraction of the coating on the diamond. Where ρd, ρc, and ρm represent the ideal density of the diamond (3.52 g/cm3), tungsten coating (19.3 g/cm3) and Cu matrix (8.9 g/cm3), respectively. The theoretical density for the un-etched diamond/Cu composite is 6.86 g/cm3 and it improve slightly with the increase of x. The closed porosity ε and the volume fraction νd of the composites can be expressed by the following equations:
ε=1−
K
⎡ 2 K d − h da − 1 ∙Vd + K d + 2 h da + 2 ⎤ m c m c ⎥ K c = Km ∙ ⎢ ⎢ 1 − K d + K d ∙V + K d + 2 K d + 2 ⎥ d K h a K h a m c m c ⎣ ⎦
where ρ is the measured density of the composites. The calculated results are listed in Table 1, the samples with etched diamond particles have a slight increase in porosity, but the increase in thermal conductivity is more significant. Fig. 10 shows the effect of surface roughness on the thermal conductivity and the interfacial thermal conductance of the diamond/Cu composites. The un-etched diamond/Cu composite had a thermal conductivity of 539 W·m−1·K−1. The conductivity of the diamond/Cu composites initially increased with the degree of etching; the maximum thermal conductivity of 602 W·m−1·K−1 was obtained after etching at 7
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diamond surface with different roughness, the increase in depth and number of etching pits increased the interfacial coupling area. Fig. 11(c–e) illustrate the mechanism of the coupling area on heat transfer, where H1 represents the total number of heat flow transfer channels. The etch pits increased the coupling areas between the two phases and provided more heat transfer channels. And according to the physical model summarized by Monachon [21], the transfer of heat could also be analogous to wave conduction. Fig. 11(e) is a schematic of single heat flow conduction, where L1, L11, L111, L2, and L21 represent the incident heat flow, the first reflected heat flow, the second reflected heat flow, the first effective heat transfer, and the second effective heat transfer. And the modified layer includes W, W2C and WC caused by the reaction between W coating and diamond during the preparation process. The etched surface absorbs the reflected energy wave two or more times, resulting in doubly or more efficient heat transfer. It could be concluded that the increase in interfacial coupling area not only increased heat transfer channels, but also enabled reflected heat flow to be transmitted again. However, it can be seen from Fig. 1(d), Fig. 5(d), Fig. 6(d) and Fig. 9(d) that excessive etching significantly damaged complete crystal planes because of the merging and expansion of the etch pits. It thus decreased the interfacial area between the two phases, greatly decreased the thermal conductivity of the diamond/Cu composite. Therefore, the thermal conductivity of diamond/Cu composites can be enhanced by controlling the surface roughness of the diamond particles.
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4. Conclusion Modified diamond/Cu composites were prepared by molten salt etching, double-coating, and hot vacuum pressing. The thermal conductivity of the composites initially increased with the degree of etching to a maximum of 602 W·m−1·K−1, then it decreased because of changes in the heat coupling area. An increase in the coupling area provided more heat transfer channels and enabled a secondary heat transfer at the diamond and Cu matrix interface. Overall, these results provide a way to design other composite materials for maximum thermal conductivity. Acknowledgments The work was supported by the National Natural Science Foundation of China (Grant Nos. 51571087, 51802088) and the Natural Science Foundation of Hunan Province (2019JJ40028). References [1] H. Guo, Y. Han, X. Zhang, C. Jia, J. Xu, Microstructure and thermophysical properties of SiC/Al composites mixed with diamond, Trans. Nonferrous Metals Soc. China 25 (2015) 170–174. [2] X. Zhang, H. Guo, F. Yin, Y. Fan, Y. Zhang, Interfacial microstructure and properties of diamond/Cu-xCr composites for electronic packaging applications, Rare Metals 30 (2011) 94–98. [3] Y. Dong, R. Zhang, X. He, Z. Ye, X. Qu, Fabrication and infiltration kinetics analysis of Ti-coated diamond/copper composites with near-net-shape by pressureless infiltration, Materials Science & Engineering: B (Advanced Functional Solid-State Materials) 177 (2012) 1524–1530. [4] E.A. Ekimov, N.V. Suetin, A.F. Popovich, V.G. Ralchenko, Thermal conductivity of diamond composites sintered under high pressures, Diam. Relat. Mater. 17 (2008) 838–843. [5] M. Yuan, Z. Tan, G. Fan, D.-B. Xiong, Q. Guo, C. Guo, Z. Li, D. Zhang, Theoretical modelling for interface design and thermal conductivity prediction in diamond/Cu composites, Diam. Relat. Mater. 81 (2018) 38–44. [6] L. Wang, J. Li, Z. Che, X. Wang, H. Zhang, J. Wang, M.J. Kim, Combining Cr precoating and Cr alloying to improve the thermal conductivity of diamond particles reinforced Cu matrix composites, J. Alloys Compd. 749 (2018) 1098–1105. [7] J. Sang, W. Yang, J. Zhu, Regulating interface adhesion and enhancing thermal conductivity of diamond/copper composites by ion beam bombardment and following surface metallization pretreatment, J. Alloys Compd. 740 (2018) 1060–1066. [8] Q. Gu, J. Peng, L. Xu, C. Srinivasakannan, L. Zhang, Y. Xia, Q. Wu, H. Xia, Preparation of Ti-coated diamond particles by microwave heating, Appl. Surf. Sci.
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Effect of surface roughening on the interfacial thermal conductance of diamond/copper composites Xinzhi Wua, Liyang Lia, Wei Zhanga, Manxin Songa,b, Wulin Yanga, Kun Penga,
T
⁎
a
College of Materials Science and Engineering, Hunan University, 410082 Changsha, China Hunan Engineering Technology Research Center for Microwave Devices and Equipment, Hunan Aerospace Chengyuan Precision Machinery Co., LTD, Changsha 410205, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface Interface Thermal conductivity Diamond Composites
Understanding the mechanism of interfacial heat transfer is the key to improved thermal conductivity of diamond/copper composites. Diamond particles are etched via molten salt to obtain different surface roughness and are subsequently coated with dual layers of tungsten and copper (Cu). Then, diamond/Cu composites are prepared by hot pressed sintering. When the (111)D surface roughness of the diamond is 11.1 μm, a maximum thermal conductivity of 602 W·m−1·K−1 is obtained, which is 12% higher than that of un-etched diamond/Cu composites. The roughness significantly increases the coupling area between the diamond and the Cu matrix, providing more heat transfer channels and a secondary heat transfer process at the diamond/matrix interface. The preparation of the high-thermal-conductivity composites is thus enhanced by controlling the surface roughness of the diamond particles.
1. Introduction
increasing the interfacial bonding area. The effect of interfacial area on the thermal boundary conductance has recently attracted attention, where it was found by molecular dynamics analysis that surface roughness could improve the thermal conductivity of composites [24]. Tan [26] and O. Beffort [27] have found that dendrite and rod-shaped carbide transition layers enhanced the thermal conductivity of diamond/aluminum (Al) composites; this was attributed to better interface bonding. However, the formation of dendrite and rod-shaped carbide transition layers also increased the interfacial area, which may have also contributed to the thermal conductivity. The thermal properties of diamond/Al composites were improved by roughening the diamond surfaces with iron‑manganese powders and nitrogen annealing [28,29]. In addition to the increased contact area between the two phases, annealing of the diamond surface led to (111) facets on the (100) surface that facilitated nucleation of carbides. However, the effect of diamond surface roughness on the heat coupling area and the thermal conductivity of diamond/Cu composites has not been systematically investigated. Here, diamond particles with different surface roughness were prepared and used in diamond/Cu composites. The thermal conductivity was improved by controlling the diamond surfaces. The effects of changes in the composite interface on heat transfer is discussed in detail.
Diamond/Cu composites have been considered the first candidate for the next generation electronic packaging materials for their excellent thermal conductivity in theory [1,2]. However, poor bonding [3] and high interfacial thermal resistance [4] between the diamond and copper (Cu) has resulted in poor thermal conductivity of the composites relative to theoretical values [5]. To overcome this problem, titanium, tungsten, chromium, and boron were coated on the diamond surface [5–13], or introduced into the Cu matrix [14–17], to form carbides. However, because of the low thermal conductivities of the carbides [18] and the difficulty of controlling the thicknesses [19,20], it was difficult to achieve the ideal thermal conductivity in the composites. The focus in these studies was to increase the interfacial thermal coupling between the two phases by the formation of carbides on the diamond surface. However, they ignored contributions from a number of interfacial heat transfer channels to the overall thermal conductivity. The interfacial heat transfer across the interface between two dissimilar materials [21–25] plays a key role in the thermal conductivity of composites. Heat transfer through the interface is realized by the coupling between carriers in the two phases. Therefore, the heat transfer efficiency is determined by the magnitude of the coupling and interfacial heat transfer channels. The former can be improved by the introduction of a transition layer, while the latter can be realized by ⁎
Corresponding author. E-mail address: [email protected] (K. Peng).
https://doi.org/10.1016/j.diamond.2019.107467 Received 25 April 2019; Received in revised form 26 June 2019; Accepted 10 July 2019 Available online 11 July 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. SEM micrographs of the diamond particle etched at different temperature: (a, a1, a11) Un-etched, (b, b1, b11) 750 °C, (c, c1,c11) 800 °C, (d, d1, d11) 850 °C,
(15 ml·L−1). The pH was then adjusted to 12 by NaOH (10 mol·L−1). The diamond particles were reacted at 30 °C for 12 h, and then washed and dried to obtain double-coated samples. During the experiment, we tested the quality change of the diamond before and after each coating. It should be pointed out that the experimental conditions of the different samples are identical except for the change of the etching temperature.
2. Experimental 2.1. Surface modification of diamond particles Synthetic diamond particles (HWD-40) with an average size of 400 μm (40–45 mesh) were purchased from Henan Huanghe Whirlwind International Co., Ltd. They were complete cubo-octahedrons with a nitrogen content of 150–170 ppm [30] and had a 1500-W·m−1·K−1 thermal conductivity. The particles were cleaned with acetone and hydrochloric acid to remove grease from the surfaces. They were then etched with molten potassium nitrate at a 1:10 diamond/potassium nitrate weight ratio, followed by heating at 750 °C, 800 °C, or 850 °C for 15 min. After cooling to room temperature, the consolidated potassium nitrate was dissolved with deionized water, and residual impurities were removed by ultrasonic cleaning. And the detail experimental process was described in our previous paper [31]. A layer of tungsten (W) was coated onto the diamond particles via vacuum diffusion. The volume ratio of particles to high-purity W powder with an average size of about 75 μm (200 mesh) was 1:1. The mixed powders were placed in a corundum crucible and the diffusion treatment was performed in a vacuum at 1.0 Pa, 900 °C for 150 min. After the tube furnace returned to room temperature, the W-coated diamond particles were extracted from the mixed powder via ultrasonic cleaning and mechanical sieving. Then, the W layer was coated with a layer of Cu via electroless plating. The typical bath chemistry includes: Cu2SO4·5H2O (15 g·L−1), Na2EDTA·2H2O (30 g·L−1) and HCHO (37%)
2.2. Preparation of diamond/Cu composites High purity copper powder (Hunan CSBLSTAR Metal Materials Co. Ltd. of China) with mean size of 45 μm (400–450 mesh) was used as the composite matrix. The Cu powders were mixed with the surface-modified diamond particles (about 38 vol%) and then placed in a graphite mold that was held in a 900 °C vacuum hot press sintering furnace (JY160-00) at a pressure of 70 MPa for 60 min in vacuum atmosphere (about 1 × 10−1 Pa). 2.3. Characterization Scanning electron microscopy (SEM) images and elemental mapping were acquired with a field-emission scanning electron microanalyzer (TESCAN MAIA3), equipped with an energy-dispersive X-ray analyzer. Raman spectra were obtained with a spectrometer (Renishaw 2000) and 514-nm laser excitation. Three-dimensional (3D) model maps and the surface roughness Rz with an accuracy of ± 0.5 μm were 2
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Fig. 2. 3D microscopic map of diamond particle etched at 800 °C for 15 min, (a) (111)D surface of diamond; (b) (100)D surface of diamond.
Fig. 3. Surface roughness change of (100)D and (111)D surfaces.
Fig. 4. Raman spectra of the diamond with different etching conditions.
acquired with a VHX-2000C super-high magnification zoom lens on a 3D microscope. The phase structure of the diamond particles was confirmed with X-ray diffraction (SmartLab, Rigaku Japan) with a CuKα radiation source. The samples used to observe the interface of the diamond/Cu composites by electrochemical polishing method. The electrolyte was H3PO4 with a volume fraction of 70%, the current density was 1A/cm2 and the corrosion time was 10 min.
The mass-average thickness of the coating for smooth diamond particles was calculated by a gravimetric method. The thermal conductivity of the diamond/Cu composites has been obtained by the formula: K = α ∙ c ∙ ρ, Where K is the thermal conductivity, α is the thermal diffusivity with an accuracy of ± 3% obtained via laser-flash analyzer (LFA467), c is the theoretical specific heat capacity obtained by the rule of mixture (ROM), ρ represents the density of diamond/Cu 3
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Fig. 5. Morphologies of diamond particles after tungsten coating with different etching degrees; (a) Un-etched, (b) 750 °C, (c) 800 °C, (d) 850 °C; (e), (f), (g), (h) are higher magnification images at 800 °C.
Fig. 6. Morphologies of diamond particles after tungsten and copper coating with different etching degrees; (a) Un-etched, (b) 750 °C, (c) 800 °C, (d) 850 °C; (e) and (f) are higher images at 800 °C.
morphologies of the raw and etched particles are shown in Fig. 1. The raw particles were cubo-octahedral crystallites consisting of six square {100} faces and eight hexagonal {111} faces with smooth surfaces. Fig. 1(b–d) shows the morphologies of particles etched at 750 °C, 800 °C, and 850 °C, respectively. The subscripts represent different magnifications. The particles etched at low temperature maintained the cubo-octahedron shape, but many triangular and square structure pits were observed on the {111} and {100} planes. It can be further seen
composites obtained by the Archimedes drainage method.
3. Results and discussion 3.1. Surface modification of diamond particles The synthetic diamond particles were etched with potassium nitrate at different temperatures (750 °C, 800 °C, and 850 °C), respectively. The 4
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Fig. 7. Morphology and elemental mapping of the coated diamond particle: (a–d) W coated; (e–h) W and Cu coated.
shown in Fig. 2 for particles etched at 800 °C for 15 min. The etch pits of triangles and squares were distributed on the (111) and (100) planes, respectively. Through modeling of the instrument, a 3D map containing specific etch pit depths (surface roughness) was obtained. The relationship between surface roughness Rz and etching temperature is shown in Fig. 3. Un-etched particles had a model height difference of 0.8–1.7 μm matching Fig. 1(a11) because of intrinsic defects. The pit depth on the (111) plane was about 1.5 times that on the (100) plane because the (111) plane was more easily etched given its lower chemical resistance [32]. Hence, the (111) roughness was greater under the same etching conditions, and it increased with temperature. Fig. 4 shows micro laser Raman spectral results. The diamond particles with various degrees of etching had only one peak at 1332.2 cm−1, which was associated with the sp3 structure, and was consistent with raw diamond. Thus, it could be inferred that the molten salt etching did not cause graphitization of the diamond or affect its intrinsic thermal conductivity. The W-Cu dual-layer coatings can increase the homogeneity and density of composite materials [33–35], and allow the Cu matrix to penetrate the etch pits more easily. The surfaces of the W-coated diamond particles were characterized with SEM, as shown in Fig. 5. The etched pits were coated outside and inside with a relatively uniform and continuous layer. The microstructure was still clearly visible, which provided a favorable basis for examining the effects of interfacial area on heat transfer. The surfaces of the Cu-W dual-layer-coated diamond particle were characterized with SEM, as shown in Fig. 6. By comparing Fig. 5 and Fig. 6, it was found that the Cu layer was relatively thick and that the etch pits had been filled. Secondly, according to Fig. 5(h) and Fig. 6(f), the Cu and W coating are dense, rough and continuous.
Fig. 8. X-ray diffraction of the diamond particles; (a) W coated, (b) W and Cu coated.
from the high magnification images, the size of the etched pits increased with the temperature. The formation of the pits was attributed to the atomic configuration of the diamond surface, distortion energy, and functional oxygen groups [31,32]. For the sample etched at 850 °C, the etching pits merged into large pits, and the edges and corners of the particles were severely etched. To obtain the quantitative roughness of the etched diamond surfaces, optical morphologies and the 3D model maps were used, as
Table 1 Thermal diffusivity α, specific heat capacities c, densities ρ, closed porosity ε, diamond volume fraction νd and thermal conductivity of the Cu matrix and the diamond/Cu composites. Samples Cu Un-etched 750 °C-etched 800 °C-etched 850 °C-etched
α (mm2·s−1)
c (J·g−1·m−1)
ρ (g·cm−3)
ε (%)
νd (%)
K (W·m−1·K−1)
111.0 191.5 200.5 214.2 155.9
0.395 0.42 0.42 0.42 0.42
8.90 6.75 6.71 6.72 6.75
0.9 1.6 2.5 2.6 2.5
– 38 38 38 38
390 539 557 602 439
5
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Fig. 9. Microstructures of diamond/Cu composites with different etching degree: (a) Un-etched, (b) 750 °C, (c) 800 °C, (d) 850 °C.
treatment. In order to quantitatively estimate the thickness of the coating, the equivalent mass-average thickness (h, nm) of ideal and smooth W coating were calculated by the following formula [36]:
h=
mw ·103 ρw ms
(1)
where mw represent the tungsten mass in the coating and m is the diamond mass. ρw is the tungsten density (19.3 g/cm3), s is the specific geometric surface area of diamond particles, which can be calculated by the following formula [36]:
s=
6 ρd Dψ
(2)
where ρd represents the diamond density (3.52 g/cm3), D is the average size of the diamond particle, the correction factor ψ of cubo-octahedron is 0.905 [36], s = 0.0047 m2/g can be calculated by the equation, and then the mass-average thickness of W coating can be obtained. The mass-average thickness of Cu can also be estimated by this method. The thickness of W and Cu coating on the un-etched diamond particles is about 170 nm and 2.4 μm, respectively, and they have a decrease with the increase of surface roughness.
Fig. 10. TC and inferred ITC of diamond/Cu composites with different etching degrees.
Fig. 7 shows the morphology and elemental distribution of the Wcoated and Cu- W dual-layer-coated diamond particles. The W and Cu were uniformity coated on the diamond surfaces. However, the elemental mapping cannot detect the existence of the W because of the thick coating of Cu. From the XRD patterns in Fig. 8, the elemental W was present as W, W2C, and WC following vacuum diffusion, whereas the elemental Cu was present as crystals. No metal oxides were detected throughout the entire process, and no diamond graphitization occurred during the heat
3.2. Microstructure of diamond/Cu composites The composite material was treated with electrochemical corrosion to more intuitively observe the interfacial structure between the diamond and the Cu matrix. Fig. 8 shows the interfacial structure of the diamond/Cu composites after different degrees of etching after W and Cu coating. A good combination of diamond and matrix was 6
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Fig. 11. Schematic diagram of etching pits and interfacial heat flux: (a-b) etching and interface coupling area; (c) Flat interface, (d) Rough interface, (e) Singlecoupling at the pits.
800 °C for 15 min. However, as the degree of etching increased, the thermal conductivity decreased significantly to 439 W·m−1·K−1, which was lower than that of the raw diamond/Cu composite. These changes could be attributed to the diamond particle surfaces at the composite interfaces. To analyze the thermal conductivity of particle-reinforced metal matrix composites, the Hasselman-Johnson (H-J) [37] model considers the effects of particle size, volume fraction, and interfacial thermal conductance. It has a wide range of applications, and can be expressed as follows:
maintained. The Cu matrix was relatively compact without porosity. Moreover, etching pits were not destroyed by hot pressing, which also may have increased the matrix/diamond bonding and strengthened the mechanical properties of the composite. 3.3. Thermal conductivity of the diamond/Cu composites The thermal properties of Cu and the diamond/Cu composites are listed in Table 1. The thermal diffusivity was a three-point average, and the calculation of the specific heat capacity did not include the coating. The theoretical density (ρ⁎) of the ideal nonporous composites was obtained by the formula [36]:
ρ∗ =
( (
1+C 1−x ρd
+
x ρc
+
C ρm
(3)
ρ ρ∗
(4)
νd =
1−x ρ · 1 + C ρd
(5)
K
) )
K
K
(6)
K is the thermal conductivity, and the subscripts c, d, and m represent composite materials, reinforcing particles, and matrix, respectively. V and a represent the real volume fraction and the radius of the diamond particles in the filler, respectively, and hc is interfacial thermal conductance. By substituting the experimental value for K into Eq. (1), hc can be calculated. The trend was consistent with the change in thermal conductivity of the diamond/Cu composite, with a hc maximum of 2.23 × 107 W·m−2·K−1 after 800 °C etching. And the results are shown in Fig. 10. The interfacial thermal conductance calculated in the H-J model is a parameter that characterizes the interface combination. The H-J equation is derived from smooth-filled particles, because the surface roughness is much smaller than the particle size, the model can be used for approximate estimation of interfacial thermal conductance were effective. Here, the diamond particle sizes and fractions were kept the same, and the surfaces were modified by the same method used to guarantee a tight coupling with the copper matrix. Thus, the uniformity of the interface per unit area was unchanged, and the only change was the diamond surface roughness. Therefore, the heat transfer per unit area (heat coupling) at the interface could be considered as a constant, and the changes in interfacial thermal conductance were caused by changes in the interfacial coupling area. Interfacial heat transfer in the diamond‑copper composites was attributed to the synergy of phonons and electrons [22,38]. Fig. 11(a–b) shows the Schematic diagram of etching pits and the coupling area of
where C is the mass ratio of the Cu to the coated diamond, and x is mass fraction of the coating on the diamond. Where ρd, ρc, and ρm represent the ideal density of the diamond (3.52 g/cm3), tungsten coating (19.3 g/cm3) and Cu matrix (8.9 g/cm3), respectively. The theoretical density for the un-etched diamond/Cu composite is 6.86 g/cm3 and it improve slightly with the increase of x. The closed porosity ε and the volume fraction νd of the composites can be expressed by the following equations:
ε=1−
K
⎡ 2 K d − h da − 1 ∙Vd + K d + 2 h da + 2 ⎤ m c m c ⎥ K c = Km ∙ ⎢ ⎢ 1 − K d + K d ∙V + K d + 2 K d + 2 ⎥ d K h a K h a m c m c ⎣ ⎦
where ρ is the measured density of the composites. The calculated results are listed in Table 1, the samples with etched diamond particles have a slight increase in porosity, but the increase in thermal conductivity is more significant. Fig. 10 shows the effect of surface roughness on the thermal conductivity and the interfacial thermal conductance of the diamond/Cu composites. The un-etched diamond/Cu composite had a thermal conductivity of 539 W·m−1·K−1. The conductivity of the diamond/Cu composites initially increased with the degree of etching; the maximum thermal conductivity of 602 W·m−1·K−1 was obtained after etching at 7
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diamond surface with different roughness, the increase in depth and number of etching pits increased the interfacial coupling area. Fig. 11(c–e) illustrate the mechanism of the coupling area on heat transfer, where H1 represents the total number of heat flow transfer channels. The etch pits increased the coupling areas between the two phases and provided more heat transfer channels. And according to the physical model summarized by Monachon [21], the transfer of heat could also be analogous to wave conduction. Fig. 11(e) is a schematic of single heat flow conduction, where L1, L11, L111, L2, and L21 represent the incident heat flow, the first reflected heat flow, the second reflected heat flow, the first effective heat transfer, and the second effective heat transfer. And the modified layer includes W, W2C and WC caused by the reaction between W coating and diamond during the preparation process. The etched surface absorbs the reflected energy wave two or more times, resulting in doubly or more efficient heat transfer. It could be concluded that the increase in interfacial coupling area not only increased heat transfer channels, but also enabled reflected heat flow to be transmitted again. However, it can be seen from Fig. 1(d), Fig. 5(d), Fig. 6(d) and Fig. 9(d) that excessive etching significantly damaged complete crystal planes because of the merging and expansion of the etch pits. It thus decreased the interfacial area between the two phases, greatly decreased the thermal conductivity of the diamond/Cu composite. Therefore, the thermal conductivity of diamond/Cu composites can be enhanced by controlling the surface roughness of the diamond particles.
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4. Conclusion Modified diamond/Cu composites were prepared by molten salt etching, double-coating, and hot vacuum pressing. The thermal conductivity of the composites initially increased with the degree of etching to a maximum of 602 W·m−1·K−1, then it decreased because of changes in the heat coupling area. An increase in the coupling area provided more heat transfer channels and enabled a secondary heat transfer at the diamond and Cu matrix interface. Overall, these results provide a way to design other composite materials for maximum thermal conductivity. Acknowledgments The work was supported by the National Natural Science Foundation of China (Grant Nos. 51571087, 51802088) and the Natural Science Foundation of Hunan Province (2019JJ40028). References [1] H. Guo, Y. Han, X. Zhang, C. Jia, J. Xu, Microstructure and thermophysical properties of SiC/Al composites mixed with diamond, Trans. Nonferrous Metals Soc. China 25 (2015) 170–174. [2] X. Zhang, H. Guo, F. Yin, Y. Fan, Y. Zhang, Interfacial microstructure and properties of diamond/Cu-xCr composites for electronic packaging applications, Rare Metals 30 (2011) 94–98. [3] Y. Dong, R. Zhang, X. He, Z. Ye, X. Qu, Fabrication and infiltration kinetics analysis of Ti-coated diamond/copper composites with near-net-shape by pressureless infiltration, Materials Science & Engineering: B (Advanced Functional Solid-State Materials) 177 (2012) 1524–1530. [4] E.A. Ekimov, N.V. Suetin, A.F. Popovich, V.G. Ralchenko, Thermal conductivity of diamond composites sintered under high pressures, Diam. Relat. Mater. 17 (2008) 838–843. [5] M. Yuan, Z. Tan, G. Fan, D.-B. Xiong, Q. Guo, C. Guo, Z. Li, D. Zhang, Theoretical modelling for interface design and thermal conductivity prediction in diamond/Cu composites, Diam. Relat. Mater. 81 (2018) 38–44. [6] L. Wang, J. Li, Z. Che, X. Wang, H. Zhang, J. Wang, M.J. Kim, Combining Cr precoating and Cr alloying to improve the thermal conductivity of diamond particles reinforced Cu matrix composites, J. Alloys Compd. 749 (2018) 1098–1105. [7] J. Sang, W. Yang, J. Zhu, Regulating interface adhesion and enhancing thermal conductivity of diamond/copper composites by ion beam bombardment and following surface metallization pretreatment, J. Alloys Compd. 740 (2018) 1060–1066. [8] Q. Gu, J. Peng, L. Xu, C. Srinivasakannan, L. Zhang, Y. Xia, Q. Wu, H. Xia, Preparation of Ti-coated diamond particles by microwave heating, Appl. Surf. Sci.
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